1. Field of the Invention
The present invention relates generally to a structure of a strained double-gated metal oxide semiconductor field effect transistor (MOSFET) having a very thin vertical silicon layer (a FIN) for the channel, referred to as a FINFET, and a method for manufacturing a strained FINFET that replaces a part of the gate of the FINFET with a stress material to apply stress to the channel of the FINFET. The stress enhances electron and hole mobility and improves the performance of the FINFET. More particularly, the SiGe part of a SiGe/Si stacked gate of a FINFET is replaced with a stress nitride film to apply stress to the channel of the FINFET.
2. Discussion of the Prior Art
Over the past twenty-five years or so, the primary challenge of very large scale integration (VLSI) has been the integration of an ever-increasing number of metal oxide semiconductor field effect transistor (MOSFET) devices with high yield and reliability. This was achieved mainly in the prior art by scaling down the MOSFET channel length without excessive short-channel effects. As is known to those skilled in the art, short-channel effects are the decrease of threshold voltage Vt in short-channel devices due to two-dimensional electrostatic charge sharing between the gate and the source/drain diffusion regions.
To scale down MOSFET channel lengths without excessive short-channel effects, gate oxide thickness has to be reduced while increasing channel-doping concentration. However, Yan, et al., “Scaling the Si MOSFET: From bulk to SOI to bulk”, IEEE Trans. Elect. Dev., Vol. 39, p. 1704, July 1992, have shown that to reduce short-channel effects for sub-0.05 μm MOSFETs, it is important to have a backside-conducting layer present in the structure that screens the drain field away from the channel. The Yan, et al. results show that double-gated MOSFETs and MOSFETs with a top gate and a backside ground plane are more immune to short-channel effects and hence can be scaled to shorter dimensions than conventional MOSFETs.
The structure of a typical prior art double-gated MOSFET consists of a very thin vertical Si layer (Fin) for the channel, with two gates, one on each side of the channel. The term “Fin” is used herein to denote a semiconducting material which is employed as the body of the FET. The two gates are electrically connected so that they serve to modulate the channel. Short-channel effects are greatly suppressed in such a structure because the two gates very effectively terminate the drain field line preventing the drain potential from being felt at the source end of the channel. Consequently, the variation of the threshold voltage with drain voltage and with gate length of a prior art double-gated MOSFET is much smaller than that of a conventional single-gated structure of the same channel length.
For FinFET CMOS (complementary metal oxide semiconductor) applications, it is beneficial to provide a structure that has the thinnest single crystal silicon Fin possible for the device body. However, this makes contacting of the source and drain regions quite difficult. Optimally, the device portion of the Fin is quite thin, with the source and drain regions being thicker, in order to facilitate silicide growth and metal contact schemes.
It is known in the present state of the art that stress can enhance electron and hole mobility in the channel of a FINFET, but it is difficult to apply a large stress in the channel of a FINFET.
Compressive longitudinal stress along the channel increases drive current in p-type field effect transistors (pFETs) and decreases drive current in n-type field effect transistors (nFETs). Tensile longitudinal stress along the channel increases drive current in nFETs and decreases drive current in pFETs if the tensile strength is in the order of 1.0 Gpa or less.